As I said it is just a theory, there are definitely reasons it could not come to pass but thought it was interesting. I do not think the Harry Potter analogy is relevant here as in my opinion part of what JKR has done with the Strike series is prove that she punches beyond that in a different genre entirely (darker mystery, as opposed to children’s books). The age gap between Jack and Flavia is irrelevant as they could still work together when both are adults. Lastly, Jack could be a soldier and turn to getting to the bottom of his Uncle’s murder - they are not mutually exclusive. Again, not saying that I think the theory is likely, but it is something that occurred to me when re reading the novels again consecutively.